The present invention relates to an electromagnetic clutch used for an automobile air-conditioner or the like and, more particularly, to an improvement in an electromagnetic clutch having a synthetic resin pulley integrally formed on a rotor.
An electromagnetic clutch of this type is disclosed in, e.g., Japanese Patent Laid-Open No. 58-196328, in which a synthetic resin driving member is integrally mounted on a rotor. More specifically, in this conventional device, an annular gear as a resin driving member is fixed on the outer circumferential surface of the rotor in the following manner. In consideration of the possibility that the rotor might be deformed if grooves are axially formed in the outer circumferential surface of the rotor by component rolling, a plurality of grooves are formed in the outer circumferential surface of the rotor by pressing the rotor into a jig having a plurality of cutting tools fixed thereto along the circumferential direction. The resin annular gear is then fitted on the outer circumferential surface of the rotor having such grooves formed therein. With this structure, while slipping of the annular gear on the rotor is prevented, the two components are integrally formed.
In such a conventional structure, however, the rotor and the resin annular gear, which are separately formed, are simply fitted together to be integrally rotatable in such a manner that the grooves formed in the rotor surface by the above-mentioned cutting tools are meshed with the projections of the resin gear. This structure requires the resin annular gear to have a considerably high strength and hence poses a problem in terms of practicability.
Under the circumstances, in order to reliably form the above-described rotor and resin driving member into an integral part, the present applicant has considered the process of forming an annular rotor having a U-shaped cross-section by cold forging, forming projections on the outer circumferential surface of the rotor by knurling, and integrally forming a synthetic resin pulley as a driving member by insert molding.
As, however, additional processing operations such as component rolling, cutting, and knurling are required to integrally form the above-described rotor and synthetic resin pulley, poor productivity cannot be avoided. Therefore, an electromagnetic clutch cannot be provided at a low cost.
In addition, limitations are imposed on the pulley diameter of an electromagnetic clutch, as the automobile manufacturers demand a reduction in size of the electromagnetic clutch. On the other hand, it is required that the thickness of a resin pulley be increased to a certain degree in order to ensure its rigidity. Consequently, the outer diameter of such a rotor is reduced, and only a small number of cutting tools can be arranged with respect to the small-diameter outer circumferential surface of the rotor. As a result, the groove/projection engaging portions of the rotor and the pulley are reduced in number, posing a problem in terms of durability.
As described above and explained in the above-mentioned Japanese Patent Laid-Open No. 58-196328, in component rolling or the like, a rotor may be deformed to interfere with a field core. Therefore, it is required that some measures be taken to solve all these problems.